1. Field of the Invention
The present invention relates generally to systems and methods for radiation dose calculation, and more particularly for radiation dose calculation within sub-volumes of a Monte Carlo based particle transport grid.
2. Discussion of Background Art
Successful human radiation therapy consists of three critically linked components: patient evaluation, treatment planning, and treatment delivery. In the last several decades, technology for patient evaluation and treatment delivery have improved dramatically, providing an array of imaging (such as CT or MRI scans) and treatment delivery devices that has, so far, outmatched the radiation therapy field's ability to fully utilize them. Full realization of the benefits of these advances requires sophisticated, accurate treatment planning.
A critical part of treatment planning is an accurate determination of dose distribution in the patient. An International Commission on Radiation Units and Measurements (ICRU) paper entitled, "Use of computers in external beam radiotherapy procedures with high-energy photons and electrons," ICRU Report 42 (1987) recommends a dose calculation accuracy goal of 2% in low dose gradient regions. Currently, however, dose calculation errors in heterogeneous media range from 3% to more than 10%, as discussed in articles by M. E. Masterson et al., entitled "Interinstitutional experience in verification of external photon dose calculations," Int. J. Oncology Biol. Phys. 21 37-58 (1991); by K. Ayyangar et al., entitled "Experimental verification of a three-dimensional dose calculation algorithm using a specially designed heterogeneous phantom," Med. Phys. 21 (2) 325-329 (1993); by C. Hurkmans et al., entitled "Limitations of a pencil beam approach to photon dose calculations in the head and neck region," Radiotherapy and Oncology 39 74-80 (1995); and by T. Knoos et al., entitled "Limitations of a pencil beam approach to photon dose calculations in lung tissue," Phys. Med. Biol. 40 1411-1420 (1995)).
Convolution and superposition algorithms can be significantly more accurate that older methods, but increase computation time considerably. Convolution and superposition algorithms are discussed in articles by A. Boyer and E. Mok, entitled "A photon dose distribution model employing convolution calculations," Med. Phys. 12 169-177 (1985); by T. R. Mackie et al., entitled "A convolution method of calculating dose for 15 MV x rays," Med. Phys. 12 188-196 (1985); by R. Mohan et al., entitled "Differential pencil beam dose computation model for photons," Med. Phys. 13 64-73 (1986); by A. Ahnesjo et al., entitled "Calculation and application of point spread functions for treatment planning with high energy photon beams," Acta Oncol. 26 49-56 (1987); by R. Mohan and C. Chui, "Use of fast Fourier transforms in calculating dose distribution of irregularly shaped fields for three-dimensional treatment planning," Med. Phys. 14 70-77 (1987); by P. E. Metcalfe et al., entitled "Modeling polychromatic high energy photon beams by superposition," Aust. Phys. Eng. Sci. Med 12 138-148 (1989); and by P. W. Hoban et al., entitled "Superposition dose calculations in lung for 10 MV photon," Aust. Phys. Eng. Sci. Med 13 81-92 (1990).
Monte Carlo dose calculation techniques, however, are widely accepted as a preferred method for dose calculation. Monte Carlo techniques are capable of accurately computing dose under almost all circumstances. For related articles authored by R. Mohan, entitled "Dose Calculations for Radiation Treatment Planning," in T. M. Jenkins, W. R. Nelson, and A. Rindi, eds., Monte Carlo Transport of Electrons and Photons, 549-471 (1988); by D. W. O. Rogers and A. F. Bielajew, entitled "Monte Carlo techniques of electron and photon transport for radiation dosimetry," in The Dosimetry of Ionizing Radiation, Vol. III, edited by K. R. Kase, B. E. Bjarngard, and R. H. Attix (academic, New York (1990), pp. 427-539; and by T. R. Mackie, entitled "Applications of the Monte Carlo method in radiotherapy," in The Dosimetry of Ionizing Radiation, Vol. III, edited by K. R. Kase, B. E. Bjarngard, and R. H. Attix (academic, New York (1990), pp. 541-620. Monte Carlo techniques can also be applied toward weapons modeling and understanding the irradiation of food.
Unfortunately, Monte Carlo calculations for a single treatment plan can take days or sometimes weeks. See A. E. Nahum, "Overview of Photon and Electron Monte Carlo," in T. M. Jenkins, W. R. Nelson, and A. Rindi, eds., Monte Carlo Transport of Electrons and Photons, 3-20 (1988). Monte Carlo algorithms currently require expert users to set up and are also very slow to execute. In addition, currently, Monte Carlo algorithms are optimized for calculating doses over an entire CT or MRI scan. Thus, when a dose calculation over only a small portion of the CT or MRI scan is desired, general purpose Monte Carlo algorithms provide much more information than is necessary, at a considerable cost in computation time.
In response to the concerns discussed above, what is needed is a system and method for radiation dose calculation that overcomes the problems of the prior art.